1. Field of the Invention
The present invention relates to a laminated sheet and a method of designing the same, and a rear projection screen and a method of manufacturing the same.
2. Description of the Related Art
Rear projection screens are generally composed of a plurality of sheet-like members such as a lenticular lens sheet, Fresnel lens sheet, front panel, and so on. The screen market requires more light-weight, low-cost, and thin screens with fine pitch while having a wide display. Further, in order to offer more sophisticated features, the rear projection screens increasingly include a multi-layered structure in which Fresnel lenses, lenticular lenses, and front panels are each laminated together. Thus, thin screens having the multi-layered structure such as a double-layered lenticular lens, a double-layered Fresnel lens, and a front panel laminated with a film are currently dominant. A screen in which a front panel is laminated with a lenticular lens on an ambient light absorbing part (black stripe: BS) so as to reduce color shift to improve contrast is proposed in Japanese Unexamined Patent Application Publication 07-248537, for example. In addition, most lenticular lenses for a liquid crystal display or digital micromirror device (DMD) are used in combination with a front plate since they are too thin to be used in a single piece. For example, a lenticular lens for liquid crystal displays may be adhered, on a lenticular exit surface, to a front plate. The exit surface of a front plate may be coated with an antireflection film. Further, a multi-layered sheet may include a combination of two or more layers of which linear expansion coefficients are greatly different so as to previously curve the sheets, as disclosed in Japanese Unexamined Patent Application Publication 2001-133886, for example. The rear projection screens having various multi-layered structures have become widely used.
The rear projection screens including a combination of laminated films, however, have the following problems. When using a combination of films, lens sheets, and front panels made of different materials, the difference in the linear expansion coefficient of each material generates thermal stress as temperature changes. The thermal stress causes warpage to distort the lens. The focal point of the screen can thereby shift; in addition, the films can be separated. This leads to deterioration of image and picture quality of the rear projection screens. Though the focal point shifts to some extent when using a single-layer structure without combining different materials, the focal point shifts more greatly if using a multi-layered structure with combined materials, making the problem of image and picture quality deterioration more serious.
One way to prevent the laminated sheet of different materials from warpage deformation due to temperature changes is to adjust the ratio of the thickness of layers in such a way that layers other than the layer with highest rigidity are as thin as possible. If one layer is significantly thicker than the other layers, for example, deformation due to thermal stress can be reduced. Another way is to adjust the ratio of the elasticity of layers in such a way that there is a soft layer of which deformation force is negligible with respect to the layer with highest rigidity. Yet another way is to substantially equalize the linear expansion coefficients of layers. It is also possible to prevent the warpage due to the thermal stress by symmetrizing the structure of the laminated sheet in the thickness direction, though it imposes many restrictions on design, such as prohibition of use of different materials on two sides of the sheet. However, the above methods place strict conditions on thickness reduction or material selection and allow very low degree of design freedom, restricting the flexible change of composition, material, and thickness of layers. For example, changing the degree of elasticity involves changing the composition and material, which is undesirable since it causes a change in the optical characteristics or there is a limitation to the entire sheet thickness in some cases to make a thin screen.
Hence, conventional rear projection screens have the problem that deterioration of image and picture quality occurs with temperature changes.